There has been a growing interest in the manufacture and use of microfluidic systems for the acquisition of chemical and biological information. In particular, microfluidic systems allow complicated biochemical reactions to be carried out using very small volumes of liquid. These miniaturized systems improve the response time of reactions, minimize sample volume, and lower reagent cost.
Traditionally, microfluidic systems have been constructed in a planar fashion using silicon fabrication techniques. Representative systems are described, for example, by Manz et al. (Trends in Anal. Chem. (1990) 10(5): 144-149; Advances in Chromatography (1993) 33: 1-66). Typically, such microfluidic devices are constructed using photolithography to define channels on silicon or glass substrates, followed by the use of etching techniques to remove material from the substrate to form the channels. A cover plate is bonded to the top of this device to provide closure. Metals may also be used to fabricate microfluidic devices. More recently, other methods have been developed that allow microfluidic devices to be constructed from plastic, silicone or other polymeric materials. In most instances, however, these techniques do not lend themselves to rapid prototyping and manufacturing flexibility. Moreover, the tool-up costs for such techniques are often quite high and can be cost-prohibitive.
Generally, the mixing of fluids in a microfluidic system is problematic, since fluid flow within these devices is not turbulent. Some microfluidic mixing devices have been constructed in substantially planar microfluidic systems having channels defined in the surface (e.g., by micromachining) of a common substrate, where the fluids are allowed to mix through diffusion (see Bokenkamp et al., Analytical Chemistry (1998) 70(2): 232-236). In these systems, the fluids mix only along a lateral interface, which is commonly small relative to the overall volume of the fluids. Thus, very little mixing occurs within a reasonable period of time.
Alternative mixing methods have been developed based on electrokinetic flow. Systems providing such utility are complicated and require electrical contacts and associated controls. Additionally such systems only work with charged fluids, or fluids containing electrolytes. Finally, these systems require voltages that are sufficiently high as to cause electrolysis of water, thus forming bubbles that complicate the collection of samples without destroying them.
Thus, there is a need for mixing devices capable of thoroughly and rapidly mixing a wide variety of fluids in a microfluidic environment. It would also be desirable for such devices to be inexpensive and simple to fabricate, yet be robust enough to mix fluids reliably both over a wide variety of flow conditions and in spite of slight variations in device assembly due to manufacturing tolerances.